The present invention relates in general to a method for manufacturing an optical waveguide preform, and more particularly to a method for controlling the outside diameter of a preform bait tube during the deposition of glass layers therein.
Optical waveguide preforms have been manufactured using a vapor deposition process in which one or more layers of glass are formed on the inner surface of a glass bait tube. In the vapor deposition process, a reaction mixture flows through the bore of the glass bait tube while heating means such as a gas burner moves longitudinally along the tube to form a moving hot zone within the tube. The reaction mixture reacts in the hot zone to form soot, which flows downstream from the hot zone. At least a portion of the soot deposits on the inner surface of the tube and becomes sintered to form a glass layer. Ordinarily, the coated bait tube has at least two compositional regions: an interior region and an exterior region. The interior region will ultimately form the core of the resultant optical fiber, and the exterior region will form the cladding around the core. The preform is usually collapsed into a smaller diameter preform or preferably into a solid cylindrical mass. The relatively large diameter cylindrical preform is then drawn into a small diameter fiber.
Fibers having out-of-round cores and fibers wherein the core is not concentric with the outer cladding surface incur inordinately high splice losses during the coupling of such fibers. Also, the launching of radiation into an optical fiber and the propagation of the radiation therethrough can be adversely affected by fibers having non-uniform geometries. An optical fiber having the desired geometrical properties of circularity and concentricity can only be obtained from a preform having the same geometry. However, even when a circular bait tube is employed, the resultant preform may posses non-uniformities introduced during the deposition process. With each glass deposition pass of the heating means, the bait tube shrinks by a small amount due to surface tension, and when a burner is employed, the burner gas forces add to the tube shrinkage. Cumulatively, this effect can deform the bait tube geometry, thereby causing it to become out-of-round.
Attempts have been made to overcome the inherent shortcomings associated with vapor deposition processes. In a first approach, pressure slightly in excess of the ambient atmospheric pressure is maintained within the bore of the bait tube. The excess pressure counteracts the tendency for collapse or distortion of the tube during the deposition as a result of the externally applied heat. To effect that result, a restriction is formed in the output end of the bait tube. The resulting restriction of the rate of egress of the gaseous mixture from the tube causes the build-up of sufficient pressure to maintain the circularity of the tube during the deposition process.
In a second approach, a conical exhaust tube and a centrally located conical flow restrictor are disposed at the exhaust end of the bait tube. A monitoring apparatus with a laser beam constantly monitors the outer diameter of the bait tube. A signal from the monitoring apparatus is fed to a controller which adjusts the longitudinal position of the conical stopper within the conical exhaust tube. The internal pressure within the bait tube is controlled by varying the area of the orifice between these two conical members.
In a third approach, the diameter of the bait tube is monitored in the region of the hot zone by a device which provides a signal to gas source. Based on the signal, the gas source introduces a gas flow into a chamber provided at the downstream end of the bait tube to control the pressure within the bait tube. In this way, the outer diameter of the bait tube is maintained at a predetermined value.
In a fourth approach, the pressure in the bait tube is directly measured and fed back to a blower, which introduces a gas flow into a chamber provided at the downstream end of the tube. The gas flow rate introduced by the blower is varied in order to maintain the outer diameter of the tube at a predetermined value.
Although these conventional approaches are generally thought to be acceptable, they are not without shortcomings. Namely, conventional manufacture techniques consider only a single variable (i.e., the internal pressure of the bait tube or, alternatively the diameter of the bait tube) during the deposition process. These single variable monitoring techniques do not, however, provide sufficient accuracy. In addition, a more dynamic control of the preform diameter is desirable during the manufacture process in which the heating means (a burner for example) makes several passes along the length of the bait tube.
The present invention reside in a method for controlling the outside diameter of a preform bait tube during a glass layer deposition process. The method involves determining a total flow rate by adding together a baseline flow rate and a flow rate adjustment. The baseline flow rate is determined from a measured temperature of a hot zone of the preform bait tube. The flow rate adjustment is determined by summing three numbers N1, N2, N3. N1 is equal to a measured diameter of the preform bait tube times a diameter constant. N2 is equal to a measured pressure of an interior of the preform bait tube times a pressure constant. And N3 is equal to an integrated diameter error times an integration constant. Finally, a gas is output at the total flow rate into one of (1) a pressure chamber that is in communication with the interior of the preform bait tube and (2) the interior of the preform bait tube.
The present invention also resides in an apparatus for controlling the outside diameter of a preform bait tube during a glass layer deposition process. The apparatus includes a gas source in communication with an interior of the preform bait tube, a temperature monitor that measures a temperature of a hot zone of the preform bait tube and generates a temperature output signal indicative of the measured temperature, a diameter monitor that measures the diameter of the hot zone of the preform bait tube and generates a diameter output signal indicative of the measured diameter, and a pressure measuring unit that measure the pressure in the interior of the preform bait tube and generates a pressure output signal indicative of the measured pressure. A control means receives the temperature output signal, the diameter output signal, and the pressure output signal, and determines a total flow rate by adding together (1) a baseline flow rate associated the measured temperature, and (2) a flow rate adjustment based on the measured diameter and the measured pressure. The control means controls the gas source to output a gas at a flow rate equal to the total flow rate.
The above and other features of the invention including various and novel details of process steps and construction will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and apparatus for controlling the outer diameter of the preform bait tube is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.